tests: Skip signal detection on Haiku

On Haiku, using 'kill -9' fromm /bin/shactually causes a process to
die with the non-standard SIGKILLTHR 15, which causes 198.sysval to
fail from the unexpected value.

* doc/m4.texi (Sysval): Skip test on Haiku.
Reported by Bruno Haible,
https://lists.gnu.org/archive/html/bug-m4/2021-05/msg00004.html
1 file changed